I've never had the luck to find an original 340. So I've never speced them myself. I've always heard they were 10-10.5 with the pop up pistons. Changing in 1972 to the lower compression. Then again you could probably pull two different motors made on two different days and come up with a different compression ratio. I don't think things were as "exact" back then.
In either case. Building an engine with 9.25:1 compression then putting aluminum heads on it is a waste of time in my books. I figured someone would jump on me saying its 8.25. Yes, its "9.25" with aluminum heads. You don't lose actual compression. I'm saying an aluminum headed engine essentially "acts" like it has 1 less point of compression than an iron engine. It doesn't build the heat like the iron headed engine does. That's the best I can explain it. And I never said dart heads weren't far superior. I'm saying superior depends on your application.
You can put lipstick on a pig, it's still a pig.